This invention relates to a process for recovering acetic acid from aqueous acetic acid solutions. More particularly, this invention relates to a process for selectively recovering acetic acid from aqueous acetic acid solutions by contacting such solutions with a solvent comprising an extractant component and a diluent component, the extractant component being selected from the group consisting of di-2,4,4-trimethylpentyl-n-octyl phosphine oxide, tri-2,4,4-trimethylpentyl phosphine oxide and mixtures thereof, and the diluent component comprising at least one high molecular weight ketone.
Numerous chemical processes use and produce aqueous process streams comprising acetic acid. As a result, various processes for recovering acetic acid from aqueous acetic acid solutions are known to those skilled in the art. Many of these processes employ distillation techniques, which have substantial equipment and energy costs associated with them. In view of the foregoing, various extraction techniques to replace and/or supplement distillation as a means for acetic acid recovery have previously been developed. For example, King, C. J., "Separation Processes Based on Reversible Chemical Complexation", in Handbook of Separation Process Technology, Chap. 15, pp. 760-74 (R. W. Rousseau ed. 1987) and King, C. J., "Acetic Acid Extraction", in Handbook of Solvent Extraction, Chap. 18.5, pp. 567-72 (T. C. Lo, M.H.I. Baird & C. Hanson eds. 1983) discuss the use of various solvents comprising an extractant component and a diluent component for extraction of acetic acid from low concentration aqueous acetic acid solutions (i.e. solutions having an acetic acid concentration of about 1.0 weight percent or less), including the use of solvents comprising trioctyl phosphine oxide (TOPO) as an extractant and various ketones as diluents.
However, many acetic acid process streams found in industrial processes are concentrated aqueous acetic acid solutions (i.e. solutions having an acetic acid concentration greater than about 1.0 weight percent). It is well known by those skilled in the art that the extraction properties of a solvent may be quite different at low and high solute concentrations, and that ordinarily the extraction properties of a solvent in high solute concentration systems cannot be inferred from the solvent's behavior in low solute concentration systems. This is due to the fact that, in liquid-liquid systems, the liquid-liquid equilibria tie lines may level out at high solute concentrations. It would therefore be advantageous to be able to use an extraction technique to extract acetic acid from both low concentration and concentrated aqueous solutions. Moreover, it would be extremely advantageous, in employing an extraction process to extract acetic acid from such aqueous acetic acid solutions, to use a solvent which readily differentiates between acetic acid and water, thereby avoiding the extraction of large quantities of water together with acetic acid, and the associated equipment and energy costs involved in downstream water separation.
It is one object of this invention to provide a process wherein the use of distillation techniques in recovering acetic acid from an aqueous solution thereof is minimized. It is a feature of this invention that acetic acid is selectively recovered from an aqueous solution thereof by contacting the solution with a solvent comprising an extractant component present in a concentration of at least 10 weight percent which is selected from the group consisting of di-2,4,4-trimethylpentyl-n-octyl phosphine oxide, tri-2,4,4-trimethylpentyl phosphine oxide, and mixtures thereof, and a diluent component comprising at least one high molecular weight ketone. This invention is advantageous in that it avoids or minimizes the use of expensive and energy intensive distillation techniques in the recovery of acetic acid from aqueous solutions thereof.
It is another object of this invention to provide a process for selectively recovering acetic acid from low concentration and concentrated aqueous acetic acid solutions, including those associated with high purity applications such as acetic acid production. It is a feature of this invention that it selectively recovers acetic acid from such aqueous acetic acid solutions.
It is another object of this invention to provide a process for recovering acetic acid from an aqueous acetic acid solution wherein the amount of water extracted from solution with the acetic acid is minimized due to the selectivity of the solvent used. It is a feature of this invention that the solvent employed selectively extracts acetic acid from an aqueous acetic acid solution. The solvent used in this invention is further advantageous in that it tends to avoid reaction with other constituents in a given process and does not accumulate therein.
Various industrial processes yield aqueous waste or by-product steams containing acetic acid, and thus employ methods for recovering acetic acid from such streams. For example, the most widely used process for the production of terephthalic acid (PTA), a purified intermediate product used in the manufacture of polyester fiber and dimethylterephthalate polymer for plastic bottles, entails the reaction of p-xylene and oxygen in an acetic acid solution. The process requires the purification of an aqueous acetic acid process stream for solvent recovery. Although initial distillation of water from the acid stream recovers most of the acid, the aqueous waste stream from the distillation operation still contains appreciable acetic acid. Traditionally, recovery of the remaining acid from the waste stream has been accomplished through azeotropic distillation, which is an energy intensive operation. It would therefore be advantageous to employ a less energy intensive means for recovering acetic acid from aqueous waste process streams in the above-described PTA process.
It is another object of this invention to provide a process for recovering acetic acid from an aqueous waste process stream in a PTA production process. It is a feature of this invention that the aqueous waste process stream is contacted with the solvent of this invention to extract acetic acid from the process stream. This is advantageous in that it avoids the use of energy-intensive azeotropic distillation to recover acid from the waste stream. This invention may be employed with additional downstream processing steps such as distillation and stripping to further purify the acetic acid recovered from the waste stream.
Another commercial process requiring large amounts of acetic acid is the process for production of cellulose acetate (CA). A widely used process for the production of CA employs an acetylation mixture consisting of acetic anhydride, acetic acid, and sulfuric acid, which is reacted with cellulose to form CA. Recovery of acetic acid is typically achieved by solvent extraction of acetic acid from water followed by distillation of the solvent to separate out acetic acid. However, the solvent typically employed also extracts water which must also be distilled.
Furthermore, usually the solvent employed has a lower boiling point than acetic acid, thus necessitating vaporization of all the solvent. However, coextraction of the water by existing solvent technology substantially increases the energy requirements of the current solvent recovery-acid purification distillation. It would thus be advantageous to minimize the amount of water coextracted in an acetic acid recovery process employed in a CA production process.
It is yet another object of the is invention to provide a process for recovering acetic acid from the aqueous acetic acid waste stream in a process for production of CA. It is a feature of this invention that only a small amount of water is co-extracted from the waste stream, thus advantageously reducing equipment and energy costs in the CA process.